The present invention relates to silicone rubber compositions having low vapor moisture transmission and more particularly the present invention relates to a one-component room temperature vulcanizable silicone rubber composition having a low vapor moisture transmission rate.
Silicone compositions and specifically one-component room temperature vulcanizable silicone rubber compositions are well known. These compositions normally comprise a silanol-terminated linear diorganopolysiloxane polymer, a filler, a functional silane cross-linking agent and a catalyst. Of course, other optional ingredients may be added to this basic composition. The resulting composition which is prepared in an anhydrous state and stored in an anhydrous state with all the ingredients mixed therein, is simply applied to a surface for a particular application such as, sealing applications, and in the presence of atmospheric moisture the silicone composition will then cure to a silicone elastomer. Such compositions are utilized for a variety of purposes such as, for instance formed-in-place gaskets but are most widely used in sealant applications and specifically in the sealing of window panes, as well as other ceramic surfaces. Such one-component room temperature vulcanizable silicone rubber compositions are quite adhesive to most substrates and cure to a silicone elastomer, which has outstanding resistance to the elements, as well as to the ozone in the air and as well as to ultra-violet rays which usually deteriorate other more conventional sealants. In addition, while such one-component room temperature vulcanizable silicone rubber compositions are adhesive to most substrates, they also have desirable tensile strength and modulus in addition to their other properties such that they will maintain their adhesion as a sealant with respect to window panes from very low temperatures such as -60.degree. F to very excessively high temperatures such as, 300.degree. F or above, that is, such silicone sealants maintain their protective properties even at excessively low temperatures or excessively high temperatures.
Recently, there has been an innovation in the construction industry to produce, instead of thermopane windows, insulated glass windows in the construction of buildings. Thermopane windows are those in which two sheets of glass are adhered to one another around the edges with a layer of air in between the glasses from which there is removed all moisture and the resulting dual window pane or thermopane has outstanding insulation resistance to the passage of thermal energy. However, such thermopane windows are excessively expensive to manufacture, as can be understood, since all the edges of the two windows have to be sealed to each other to prevent the passage of any moisture into the air pocket that is located between the two window panes.
Accordingly, a new innovation has been brought into the manufacture of such thermally insulated window panes and that is, the insulated glass design. In such insulated glass design two window panes are taken and they are sealed at their edges by a sealant which may be, for instance, a polysulfide sealant. The window panes are then installed in a building and particularly a high rise building and are further sealed to the window frame by said polysulfide sealants. Unfortunately, it has been found that such insulated glass construction with such prior art sealants allow more moisture than is desired to be transmitted through the sealant into the air space between the window panes. Accordingly, as a result of this, too much moisture migrates through the sealant into the air space between the two window panes and the window tends to fog up as a result of the moisture particles in the air space between the two window panes condensing out against the window pane. This has been found to be the case, even when conventional one-component or two-component room temperature vulcanizable silicone rubber compositions were utilized as sealants.
One solution to this problem was to utilize butyl tape which has a low moisture vapor transmission rate as an initial sealant in the edges of such insulated glass panes in which tapes there are located micro sieves or silica gel and thereover apply the conventional sealant or conventional silicone sealant to seal or prepare the insulated glass pane from the two or three individual glass window panes. Unfortunately, such conventional silicone sealants or other conventional sealants even with the butyl tape with the dessicant in the space over which the sealant was applied still resulted in a larger than desirable moisture vapor transmission rate into the air space between the window panes. As a result, in some cases, these types of constructed insulated glass panes would tend to fog up when the unit was tested and cycled at temperatures going from -60.degree. F to 150.degree. F. In addition, the use of silica gel along with the butyl tape and the conventional sealant would unduly increase the cost of the construction of the insulated glass. Accordingly, it was highly desirable to produce a silicone sealant which would have a moisture vapor transmission rate considerably below that of conventional silicone sealants which moisture vapor transmission rate for conventional silicone sealants is in the range of 40 to 50 grams per square meter in 24 hours, through a layer of silicone material varying anywhere from 60 to 75 mills thick. Low moisture vapor transmission rate silicone sealants were desired so that they could be utilized over butyl tape with the micro sieves or silica gel to produce insulated glass panes which would resist the passage of moisture into the air pocket between the glass panes and allow the glass panes to function at excessively low temperatures without fogging up, Especially, such glass panes would be able to pass the cycling test mentioned previously wherein such insulated glass panes are cycled from -60.degree. F to 150.degree. F and wherein during such cycling periods the window pane would not fog up.
In a most advantageous preferred embodiment in the construction of such insulated glass panes, such low moisture vapor transmission rate silicone sealants could be applied over butyl tape with dessicants and then the insulated glass pane would then be again sealed with such low moisture vapor transmission rate silicone sealants against the window frame to allow for construction of an insulated glass pane which was considerably below the cost of a thermopane and which at the same time would function as well as the thermopane without excessive transmission of vapor from the atmosphere into the air space between the glass panes of the insulated glass unit and as such the insulated glass unit would not fog up at excessively low temperatures and visibility through it would not be impaired as was the case with some prior art insulated glass panes. Two-component room temperature vulcanizable silicone rubber compositions which have these properties are those described in the copending application of Alfred H. Smith, Docket 60SI-89 -- entitled Low Vapor Transmission Rate Silicone Compositions.
Further, the present invention results in a silicone sealant with excellent weatherability and low moisture vapor transmission rate as compared to the other sealants which might have low moisture vapor transmission rate but not have good weatherability and they lose their strength after a few years.
However, two-component compositions while having the advantages of a faster cure rate than one-component compositions and being capable of having a deep section cure have several disadvantages with respect to one-component. However, in one-component room temperature vulcanizable silicone rubber compositions since all of the necessary ingredients are already mixed there is no necessity for obtaining a mixing or dispensing apparatus for applying the composition to a substrate. Accordingly, such one-component room temperature vulcanizable silicone compositions can be dispensed on site wherever the insulated glass is used or can be utilized to seal into place a third window pane on top of the two window panes in the insulated glass construction. However, one component systems have the disadvantage that in some cases they are slower to cure than the corresponding two-component system set forth in the copending Smith case. On the other hand, as pointed out previously, since one-component is already mixed in the caulking tube from which the sealant may be applied, there is no need for mixing apparatus. The composition is void-free and it does not have any bubbles which might be forced into the two-component system while it is being mixed from the mixing apparatus. Accordingly, it is much easier to dispense and apply than is the two-component system. Accordingly, it is highly advantageous to have a one-component room temperature vulcanizable silicone rubber composition with a low moisture vapor transmission rate such that it can be applied on site or any time to produce or repair insulated glass panes or if any other application where a low moisture vapor transmission rate sealant is desired.
Accordingly, it is one object of the present invention to provide for a one-component room temperature vulcanizable silicone rubber composition which has a low moisture vapor transmission rate.
It is another object of the present invention to provide for an already mixed room temperature vulcanizable silicone rubber composition which has a low moisture vapor transmission rate which can be applied and adheres to many substrates and can cure upon being exposed to atmospheric moisture. It is an additional object of the present invention to provide for a one-component room temperature vulcanizable silicone rubber composition which has a low moisture vapor transmission rate and which can be applied to seal and form insulated glass panes.
It is yet an additional object of the present invention to provide for a process for producing a one-component room temperature vulcanizable silicone rubber composition which has a low moisture vapor transmission rate.
These and other objects of the present invention are accomplished by means of the disclosure set forth hereinbelow.